Proteinaceous materials are used for a wide variety of applications. One of the predominant useful properties is their ability to dissolve in aqueous solutions and yet form a solid matrix which is permeable to aqueous solutions upon drying. These properties have been exploited for many generations in the field of photographic sciences and proteinaceous materials are still widely used as a binder for harbouring silver halide grains in the photosensitive layer of most photographic films.
Formation of a solid matrix is typically considered to be a result of inter-and intra-molecular hydrogen bonding within both the crystalline and amorphous regions of proteinaceous materials. If only the natural hydrogen bonding is employed, the wet strength of the matrix is typically insufficient for use in a photographic film. Therefore, it is common practice to add a crosslinking agent, also known as a hardener, to a protein material when used for photographic layers.
Hardeners are chosen, in part, for their ability to link one group on a protein molecule with another group on the same, or different, protein molecule. The linking generates a three-dimensional network of proteinaceous material. This three-dimensional network has sufficient wet strength to swell during processing without detrimental effects on the silver halide grain harboured therein. Another important aspect of the three-dimensional network is an ability to allow solution to permeate freely during the photographic processing steps of development, fix (or bleach) and wash. It is imperative that the solution which freely permeates the matrix is not stongly absorbed. This is particularly important for photosensitive elements since they must often be capable of transiting the photographic processing steps of development, fix, wash and dry in 20-120 sec.
Two broad classes of hardeners are known in the art. One class reacts with two proteinaceous groups and then becomes an integral part of the resulting bond. Examples of this type of hardener are legion in number and include aldehydes, triazines, chromealum and the like. A second class of hardener is thought to activate one group of a protein molecule, typically a carboxyl group, and thereby facilitate reactivity with a second, typically amine, group of a protein molecule. Reaction with the second group of proteinaceous molecules is typically thought to be a nucleophilic attack which displaces a derivative of the hardener. The resulting bond does not include the hardener but instead is a linkage involving; only chemical elements which were integral to the protein material prior to hardening. This latter type of hardener is typically referred to in the art as a "peptide coupler" since they act to form a peptide bond by fusing existing groups of a protein molecule. The hardener molecules of the present invention are considereds to be in the class of hardeners known as peptide couplers.
Peptide couplers are well known in the art and the examples are legion in number. One particularly advantageous class of peptide couplers is the 1,3-bis-carbamoyl imidazolium compounds described in U.S. patent application Ser. No. 07/817,692 filed Jan. 7, 1992. The 1,3-bis-carbamoyl imidazolium compounds offer several advantages over previous hardeners. Particularly advantagous is their solubility in aqueous solution, their stability towards decomposition, and their ability to react in a time frame which is convenient for coating and drying gelatin.
While 1,3-bis-carbamoyl imidazolium compounds are advantageous over previously known hardeners there is still a need to improve the art of hydrophilic colloid hardening. One aspect of the 1,3-bis-carbamoyl imidazolium compounds which is particularly troublesome is the synthetic procedure. The reaction involves the use of two equivalents of carbamoyl chloride derivatives. Many carbamoyl chloride derivatives are known carcinogens. One skilled in the art of chemical sciences would realize the advantage of reducing, or eliminating, the need to use such compounds in an industrial chemical environment.
Yet another disadvantage with available imidazolium couplers is the high level of water absorption of the resulting crosslinked film. As mentioned previously, excess water absorption is detrimental to a photographic element and it is important that water absorption to the matrix remain low while at the same time allowing the processing solutions to permeate freely. There is an ongoing need in the art for a hardener which affords lower water absorption properties. At the same time, this decreased water absorption must be accomplished without compromising other properties such as permeability, strength and the like.